The present invention is directed in general toward method and apparatus for recovering a digital data signal which has been phase shift modulated onto a carrier signal and, more particularly, toward method and apparatus which advantageously uses a microprocessor to synchronize the operation of a phase shift demodulator.
Transmission of digital data by varying a characteristic of an alternating current (AC) carrier signal, such as its amplitude, phase or frequency, commonly referred to as amplitude, phase and frequency modulation, respectively, is widely practiced. Intrinsic to this mode of communication is the need to establish appropriate timing signals at the receiver. For synchronous detection, a local oscillator signal, which is in phase with the received carrier signal is necessary. A local baud signal, which is in phase with the received data units, is also necessary.
Many prior art methods rely primarily upon analog circuitry to provide the local oscillator and local baud signals. For example, U.S. Pat. No. 4,143,322 to Shimamura discloses a system for recovering a data signal which has been phase shift modulated onto a reference waveform wherein an analog locked loop is used to provide the local oscillator signal. Other prior art devices, such as that disclosed in U.S. Pat. No. 4,475,217 to Hughes, employ both digital and analog circuitry to provide the local oscillator and baud signals.
However, with the increasing availability and efficiency of micro-processing devices, it is becoming more and more desirble to employ these devices to perform receiver functions, e.g., to provide the local oscillator and baud signals. Further, many prior art receivers are used in applications where a microprocessor is available to perform functions in addition to those for which it is provided. See e.g., U.S. Pat. No. 4,335,303 to Phillips et al. (1982) and U.S. Pat. No. 4,230,913 to Brothers et al. (1980). In such cases, adapting an available microprocessor to provide the local oscillator and baud signals would allow for elimination of those devices now providing these signals.
It would be advantageous, therefore, if receivers such as the ones discussed above could be integrated with an available microprocessor to allow the microprocessor to provide the local oscillator and baud signals. This would result in improved reliability due to a decrease in the total number of components of the system. Further, such a device would prove less expensive to produce than prior art devices as the number of necessary discrete components would be reduced.
Additionally, by using a microprocessor to synchronize the receiver, more synchronization signals can be provided without the need to provide additional components to produce these signals. As a result, more sophisticated operation of the analog components and consequent increase in accuracy is possible without a consequent decrease in reliability or increase in production expense.